DISCUSSION ON PAPERS BY REVELL, SWANSON, 

 KIRBY-SMITH, LANE AND DESCHNER 



L. H. Gray : ' Contact first ' hypothesis — I think it is extremely interesting tliat Reveli 

 has feh compelled to return to the view that exchanges only occur between chro- 

 matids at points at which they lie in contact, in order to account for his observations 

 on the aberrations induced in Vicia chromosomes by diepoxide. Over twenty years 

 ago this view was advanced in connection with the production of exchanges by 

 ionizing radiation, and so far as I know, the alternative hypothesis that exchanges 

 arise by union between chromosome ends which result from independently produced 

 breaks was adopted in preference to the 'contact first' hypothesis when it was shown 

 that exchanges resulting from the irradiation of Drosophila sperm increase as the 3/2 

 power of the dose. Subsequent studies with Tradescantia, Vicia and many other 

 materials have, so far as I know, always yielded the result that exchanges induced 

 by X- or gamma rays increase more rapidly than the first power of the dose, and 

 often as the square of the dose. 



In one respect the two hypotheses do not differ very greatly, for it was found neces- 

 sary by Lea and Catcheside, in developing quantitatively the generally accepted 

 view of interchange formation, to assume that in Tradescantia exchanges rarely take 

 place between breaks which are more than 1 micron apart at the time of their 

 formation, and it might become a rather fine point to distinguish between this and 

 actual contact. There is, however, one big and crucial difference, as Reveli has 

 pointed out, namely, that while the usually accepted hypothesis supposes that a 

 chromosome may be broken anywhere along its length, Reveli supposes that break- 

 age is secondary to exchange and can only occur at places at which chromatids are 

 already in contact. These are two hypotheses between which it should not be too 

 difficult to decide by experiment. 



It is clear, in the case of both chemically induced breaks and those induced by 

 ionizing agents, that many steps intervene between the acts of initiation and the 

 definitive formation of the aberrations, and in my view we already know of a number 

 of differences between aberrations induced by diepoxide and those induced by 

 ionizing radiation which make it clear that the steps cannot be identically the same 

 in the two cases. This is shown {a) by the selective localization of aberrations 

 induced by diepoxide (as well as by some other chemical agents) within restricted 

 regions of the chromosome, by contrast with the more or less random distribution of 

 radiation-induced aberrations, and {b) by the fact that in the case of Vicia meristem cells 

 studied by Reveli, the chemical agent is most effective when given during the first 

 half of interphase, whereas ionizing radiation is most effective in the second half 

 of interphase. I would suggest, therefore, that the hypothesis which we adopt as 

 to the mode of action of each agent should be that which best accommodates the 

 facts known to us about the aberrations produced by that particular agent, without 

 any attempt to arrive at a single hypothesis which would be applicable to all 

 agents. 



For myself, I find no compelling reason at the present time to abandon the accepted 

 view of aberration production by ionizing radiations. In the case of diepoxide, 

 Reveli has been led by the high proportion of aberrations which involve the hetero- 

 chromatic regions of the long M chromosome, and from the approximate linear 

 relation between exchanges and dose, to adopt the 'contact first' hypothesis for the 

 chemical agent. By the same token, the random distribution of breaks and the 

 square dose law for exchange production indicate a two-particle process for X- and 

 gamma radiation. I agree with Reveli that the two electrons might not initiate 

 breaks in two different chromosomes ; it might be that even when two chromatids 

 are lying in contact an amount of energy requiring the co-operative action of at 



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